Exemplary embodiments relate to a semiconductor memory device and a method of programming the same.
There is an increasing demand for nonvolatile memory devices which can be electrically programmed and erased and can retain its data while power is off. In order to develop high-capacity memory devices capable of storing a large amount of data, techniques for high integration of memory cells are being developed. A nonvolatile memory device includes a plurality of memory cells coupled in series to form a cell string. A plurality of cell strings forms a memory cell array.
The string of the nonvolatile memory device has a structure in which the plurality of memory cells is coupled in series between a bit line and a source line. Due to this string structure, the number of contacts of the bit line and the source line is reduced and the size of the memory cell can be reduced, thereby being capable of implementing high-capacity memory. However, when the size of the memory cell is reduced, there is a disadvantage in that the access speed is slow because the cell current of the memory cell is very small.
In this nonvolatile memory device, one memory cell can store data of 2 bits or more in a multi-level form, unlike a typical memory device. That is, the memory cell can store data of several states by having different threshold voltage through control of the amount of electric charge charged in the floating gate of the memory cell. This multi-level cell program method is being widely used because it can significantly increase the capacity of the memory cell. In the multi-level cell program method, data of two or more logical pages is programmed into one physical page. When data of several logical pages is programmed into one physical page, the time that it takes to perform a program operation is long and a process of performing the program operation is complicated. Furthermore, the width between threshold voltage distributions is narrowed because the number of the threshold voltage distributions is increased. Accordingly, there are problems in that there is a difficulty in reading data because of a reduced read margin, and an interference phenomenon in which neighboring memory cells influence each other during a program operation.